I've heard bad things about Costco tires - that they're rejects and drive very poorly. Our dealer actually demonstrated the difference between an OEM and Costco tire with the same identification # for us - the OEM was nice and firm with more tread, the Costco one squishy with less tread. Granted, it could just be the dealer trying to sell us tires, but I know they don't mark up the OEM ones they sell, so it seemed legit. So, take from it what you will.
